It is known to perform switch-off of the overcurrent only by opening a main relay. Furthermore, a fuse is often installed in the high-voltage path which is to provide redundant protection against overcurrent and in the case of short circuits.
Such a battery system is known from Japanese Patent Publication No. JP 2008-193 776 A1, for example. The battery system comprises battery blocks which are connected in series via a fuse. In addition, the battery system comprises relays which are connected to the output side of the battery. In this battery system, the fuse will blow out and switch off the current in the battery when an excessive current flows into the driving battery. Furthermore, a control circuit which controls the relay switches off the relay for cutting off the current. The control circuit of the battery system is provided with a current detection circuit for detecting the current of the battery, wherein the charging and discharging of the battery is controlled on the basis of the detected current. The current detection circuit detects the current within the normal battery charging and discharging range, e.g. at or beneath 200 A in a battery system of a vehicle. If a higher current than this one flows in the battery, especially when an abnormally high current flows in the battery, the relays are switched to “OFF” in order to interrupt the battery current.
In the case of a defect in the system, either as a result of a short-circuit or an error in the power electronics outside of the battery or by component defects, currents above the normally permitted or specified current range can occur. Fuses used in such battery packs have disadvantageous properties which limit the use for cutting off the overcurrent. Fuses must be able to bear the operating current range without ageing and without blowing out too early. In order to ensure this, the fuse element must be designed accordingly. This leads to a bottom switch-off current limit (minimum switchable current) for fuses. The fuse cannot be made to blow out beneath this bottom switch-off limit. Currents slightly beneath this bottom threshold value can lead to excessive heating and melting of the ambient parts up to the formation of fires.
On the other hand, relays which are available for battery systems of the kind mentioned above have disadvantageous properties in the switch-off capability of the current. As a result of the switch-off of the high current, the contacts can be damaged to such an extent that in the case of reactivation the contact resistance is so bad that overheating of the relay during further operation and therefore a distinct decrease in performance of the entire system may occur. Furthermore, the contact material is distributed in the contact chamber during a switch-off in the high-current range due to the strong arc, which brings the insulation resistance of one contact pole of the relay to the other beneath the permitted threshold. This leads to the likelihood that voltage can be tapped outside of the battery despite the fact that the switching relay is open.
A battery system with a fuse connected to the battery is further known from German Patent Publication No. DE 10 2009 053 712 A1, which fuse will blow out during the flow of excessive current. A relay is further connected to the output side of the battery. An excessive battery current can be detected and the relay can be controlled via a current interruption circuit. The current interruption circuit is connected to a timer section which determines a delay time for the tripping of the relay. The fusing current of the fuse will be set to a lower level for the delay time of the timer section than the maximum interruption current of the relay and higher than the maximum permitted battery, charging and discharging current. In a situation in which the excessive current which is greater than the maximum interruption current of the relay flows through the battery, the fuse will blow out during the timer delay time and the current interruption circuit switches the relay from “ON” to “OFF” when the delay time has expired. The fuse will blow out during the delay time in order to interrupt current at a current which is large enough to fuse the relay contacts. An excessive current which does not allow the fuse to blow out during the delay time would therefore be a current which does not allow the relay contacts to fuse, which is why the relays can be tripped after the delay time.
It is disadvantageous that the level of the excessive current is not detected and that the relay is tripped only after the expiration of the delay time. Although the delay time is only 0.3 seconds, excessive heating of the ambient parts can occur at current strengths which are larger than the maximum operating current strength, but only slightly lower than the tripping current strength for the fuse, as a result of which local thermal overloads cannot be excluded.